# [gmx-users] Question about the Marrink's CG model

XAvier Periole x.periole at rug.nl
Mon Sep 14 09:51:21 CEST 2009

On Sep 9, 2009, at 6:01 PM, Li Yang wrote:

> Dear GMXers:
>
> I'm sorry to disturb you.
>
> I have a question about the Marrink's coarse-grained model, Marrink,
> et al. Coarse Grained Model for Semiquantitative Lipid Simulations.
> J. Phys. Chem. B 2004, 108, 750-760.
>
First of all you should upgrade your model to the new version of this
model (the MARTINI CG model).
Marrink et al JPC/B 2007, 111, 7812-24
Monticelli et al JCTC 2008, 4, 819-34
> In the Marrink's CG model, the electrostatic Coulombic potential is
> computed via cutoff scheme. In the original paper, the cutoff method
> is employed "with relative dielectric constant \epsilon=20 for
> explicit screening". Does it mean the parameter of force field is
> only customized for the cutoff method? As known that the Ewald
> method is better than the cutoff scheme for the long-range
> electrostatic interaction. I want to know whether the Ewald method
> is also OK for this model? How can I test it? I have simulated a
> similar system via two method respectively, which includes a big
> charged nano-scale particle (~6nm). But I find that the results of
> cutoff scheme are quite different from the PME's results. I don't
> know which one is right.
The model per se is parameterized  using epsilon=15 (new version) and
a cutoff scheme.
This is what people should use, as if you change it you will alter the
thermodynamics
properties of the model and thereby modify it.
Then some test have been made with PME for the electrostatic for
bilayer systems.
Although some phenomenon does not appear in the same time scale the
overall
properties of a bilayer are not affected significantly. If you choose
to use PME
with MARTINI, you should make some test on your specific system to see
whether
it affects the behavior of the model.

Note that PME is good at some things a cutoff scheme is bad but it
introduces others
(effect of periodicity on small system, increased computational
cost ...).
You should find some literature on this.

XAvier.
>
> Another question is how to get the relative dielectric constant for
> a CG model. I'm not familiar with this topic, but I think it is the
> key of the above question perhaps. Yet, there are not some details
> in the original paper. In some other paper about different CG
>
>
> Thank you very much.
>
> Li Yang
>
>
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